The phrase "most people mean the wrong thing by this phrase" makes no sense. A phrase means what most people mean by saying it, or understand by hearing it. So,

The phrase "most people mean the wrong thing by this phrase" makes no sense. A phrase means what most people mean by saying it, or understand by hearing it. So, "the exception that proves the rule" is, as its main modern meaning, a joking way of admitting that a rule (especially one that the speaker had argued for) is not actually a universal rule, while maintaining that it generally holds true. The examples of legal signs and so on are a more specific, technical, meaning that is only used in certain contexts, such as actual legal proceedings or at least informal discussions about laws or contract terms. When /u/flkiwi above said this phrase, they obviously meant it in the joking sense I gave, and which they had actually explained above. They agree that, in general, lawyers make bad CEOs, but they also personally know of exceptions. This is not "wrong usage", as proven by the fact that everyone who read the comment understood exactly what they meant. This whole thing reminds me of the people who complain about the use of literally as an amplifier instead of for its primary meaning as "wrong", with seemingly no understanding of how flourishes and rhetoric work (nor even of the history of words like "very", which used to be quite similar to "literally" a long time ago).